scale video filter improvement

Description

Could "scale" video filter be improved in such a way that users could define X:Y as a maximum sized box in which the input video should be scaled, keeping aspect ratio? Something like: "-vf scale=640:480:keep_aspect_ratio"

This would work the same as "-vf scale=640:-1" when the input video has got width > height and "-vf scale=-1:480" when the input video has got height > width.

Right now, without this feature, users have to use either ffprobe or ffmpeg to first determine which dimension is greater and based on that, they would launch either 640:-1 (if width is greater than height) or -1:480 (when height is greater than width). But with this feature, users would be able to do it all with one simple command.

Maybe a simple flag could be added, so that it could be written like this: "-vf scale=640:480:flags=keep-ratio" or something similar.